Various packaging techniques have been used to build a load of unit products and subsequently wrap them for transportation, storage, containment and stabilization, protection and waterproofing. Products may be stacked as a load on a pallet to simplify handling of the products. The load is commonly wrapped with stretch wrap packaging material. One system uses stretch wrapping machines to stretch, dispense and wrap stretch packaging material around a load. Stretch wrapping can be performed as an inline, automated packaging technique that dispenses and wraps packaging material in a stretch condition around a load to cover and contain the load. Stretch wrapping, whether accomplished by a turntable, rotating arm, or vertical rotating ring, typically covers the four vertical sides of the load with a stretchable film such as polyethylene film. In each of these arrangements, relative rotation is provided between the load and the packaging material dispenser to wrap packaging material about the sides of the load.
Such a stretch wrapping apparatus performs admirably in accomplishing its intended goal of wrapping a load with a stretched web of film. However, the procedure of threading the film web through the series of rollers in the film web dispenser prior to operation of the stretch wrapping apparatus has been found to be time consuming and difficult. Conventional stretch wrapping apparatuses employ a film web dispenser having two closely spaced rollers to stretch between them a web of thermoplastic stretch film packaging material.
It is preferable that a stretch wrapping apparatus with pre-stretch rollers have the following features. The upstream and downstream pre-stretch rollers should be closely spaced during operation to prevent neckdown of the film web as it is being substantially stretched in the dispensing direction. Since the forces exerted on a wide web of stretch film between the pre-stretch rollers is very significant, in the order of 50 pounds of force, the frame and the mounting of the pre-stretch rollers should be required to be very sturdy. The surface of the pre-stretch rollers should be covered by the stretch film over a sufficient extent such that the stretch film does not slip on the pre-stretch rollers. The rotational inertia of the pre-stretch rollers should not be so high as to require excessively high power to vary their velocity. To accommodate these requirements, a series of idler rollers are used to support the stretch film proximate to the pre-stretch rollers and maintain the tack side of the film web facing the pre-stretch rollers. As a result, the film web follows a tortuous path around the closely spaced pre-stretch and idler rollers. The downstream pre-stretch roller has a faster surface speed than the upstream pre-stretch roller. This stretches the packaging material between the upstream and downstream pre-stretch rollers in the dispensing direction.
The increased numbers of rollers in the film dispensers for pre-stretch and the close spacing of such rollers with their effective interconnection for pre-stretching the film web prior to dispensing the film web on the load makes it very difficult to thread the film web through the pre-stretch portion of the film dispenser. Before operating a stretch wrapping apparatus having pre-stretch rollers, it is necessary to thread the stretch film along the tortuous path. This threading operation is difficult and time consuming since, due to the close spacing of the rollers, an operator cannot manually carry the stretch wrap packaging material between the rollers. Rather, the operator must feed the stretch wrap packaging material from one side of each roller nip with one hand and pull the stretch film from the other side of the roller nip with the other hand. This procedure is rendered considerably more difficult because of the large widths of the stretch wrap packaging material which are used commercially. Such webs generally have a width in the range of 10 to 30 inches. Other problems in threading resulted from the fact that the operator was required to place his fingers proximate to the nip of the rollers and blindly feed the film from the rear of the frame which carried the rollers, which made the threading operation difficult and more time consuming.
This threading problem was addressed in U.S. Pat. No. 4,747,254, the entire disclosure of which is incorporated herein by reference. This earlier arrangement reduced the difficulty of threading a web along a tortuous path by using a retractable leader which ran generally parallel to the path of the film web around the rollers to lead the film along the path. However, this arrangement required the leading end of the film web to be attached to the leader and the leader to be slowly pulled through the tortuous path while rotating the pre-stretch rollers, releasing the film web from the leader and retracting the leader.
Pre-stretch roll carriages include at least two coated pre-stretch rollers and additional rollers which are necessary for achieving the necessary film path. The pre-stretch rollers are mounted on shafts through bearings on each end. One end has a sprocket or pulley for connecting the rollers to each other and to the drive motor. The guiding rollers are also supported at both ends. Many prior art pre-stretch roll carriages “split” either vertically or horizontally to facilitate film threading around these multiple rollers. Examples of such prior art can be seen in numerous issued patents. Virtually all prior art pre-stretch roll carriages utilize a “W” threading pattern around the pre-stretch rollers which rotate in the same direction.
FIGS. 1A and 1B show a conventional pre-stretch roll carriage in the closed position and open position, respectively. As shown, a roll carriage frame 8 includes first and second upper frame portions 10a, 10b, first and second lower frame portions 12a, 12b, and first and second side frame portions 14a, 14b. The first frame portions 10a, 12a, 14a form a first side of the frame 15a and the second frame portions 10b, 12b, 14b form a second side of the frame 15b. The first and second sides of the frame 15a, 15b are pivotable relative to one another around a hinge 17. This “split frame” is necessary to permit threading of the film between the first and second sides of the frames and the rollers supported therein.
The roll carriage 8 supports two pre-stretch rollers 22, 24. The pre-stretch rollers 22, 24 are mounted on shafts and supported by first upper frame portion 10a and first lower frame portion 12a. The roll carriage 8 also supports idle rollers 28a, 28b, 28c. Each idle roller is supported by second upper frame portion 10b and second lower frame portion 12b. To thread the roll carriage 8, the film 11 is pulled between hinge 17 and over pre-stretch rollers 22, 24 while the roll carriage is in the open position. When the roll carriage is then closed, the film 11 is positioned between the pre-stretch rollers 22, 24 and the idle rollers 28a, 28b, 28c in a “W” pattern.
An attempt to simplify the threading process is exemplified in very early pre-stretch roll carriage produced by Lantech. The roll carriage utilized two pre-stretch rollers cantilevered and supported only at the top of the roll carriage, such that the bottoms of the rollers were unsupported and unconnected to any structure. This prior art device utilized an “s” threading pattern around the pre-stretch rollers, which rotated in opposite directions. This roll carriage could be threaded from the bottom without the necessity for additional idle rollers or mechanical split of the roller supports. This design was unsuccessful due to very significant problems in dealing with the forces exerted on the unsupported rollers. The forces caused deformation of the rollers and the mis-alignment of the gears used to maintain relative speeds of the two pre-stretch rollers.